Storage devices using semiconductor elements are broadly classified into two categories: a volatile device that loses stored data when power supply stops, and a nonvolatile device that holds stored data even when power is not supplied.
A typical example of a volatile storage device is a DRAM (dynamic random access memory). A DRAM stores data in such a manner that a transistor included in a storage element is selected and electric charge is held in a capacitor.
When data is read from a DRAM, electric charge in a capacitor is lost according to the above-described principle; thus, another writing operation is necessary whenever data is read out. Moreover, a transistor included in a storage element has a leakage current and electric charge flows into or out of a capacitor even when the transistor is not selected, so that the data holding time is short. For that reason, another writing operation (refresh operation) is necessary at predetermined intervals, and it is difficult to sufficiently reduce power consumption. Furthermore, since stored data is lost when power supply stops, an additional storage device using a magnetic material or an optical material is needed in order to hold the data for a long time.
Another example of a volatile storage device is an SRAM (static random access memory). An SRAM holds stored data by using a circuit such as a flip-flop and thus does not need a refresh operation. This means that an SRAM has an advantage over a DRAM. However, cost per storage capacity is increased because a circuit such as a flip-flop is used. Moreover, as in a DRAM, stored data in an SRAM is lost when power supply stops.
A typical example of a nonvolatile storage device is a flash memory. A flash memory includes a floating gate between a gate electrode and a channel formation region in a transistor and stores data by holding electric charge in the floating gate. Therefore, a flash memory has advantages in that the data holding time is extremely long (almost permanent) and refresh operation which is necessary in a volatile storage device is not needed (e.g., see Patent Document 1).
However, a gate insulating layer included in a storage element deteriorates by tunneling current generated in writing, and thus the storage element stops its function after a predetermined number of writing operations. In order to reduce adverse effects of this problem, a method in which the number of writing operations for storage elements is equalized is employed, for example. However, a complicated peripheral circuit is needed to realize this method. Moreover, employing such a method does not solve the fundamental problem of lifetime. In other words, a flash memory is not suitable for applications in which data is frequently rewritten.
In addition, a high voltage is necessary for holding of electric charge in the floating gate or removal of the electric charge, and a circuit for generating high voltage is also necessary. Further, it takes a relatively long time to hold or remove electric charge, and it is not easy to perform writing and erasing at higher speed.